Conventional optical apparatus for detecting flaws, stains and holes in web material, such as is described in British Pat. No. 1204951, includes a plurality of photodetectors disposed in a parallel configuration to detect reflected or transmitted light from the web material. A signal from each detector is amplified in a separate channel, and the amplified signal is compared with a threshold level. The result of the comparison indicates the presence or absence of an abnormally high signal level which would indicate a flaw such as a stain or hole in the region of the web material adjacent to that detector.
This technique was refined in the apparatus of British Pat. No. 1387320, which was particularly useful for detecting stains and flaws on the surface of moving sheet metal. An amplified signal from each of an array of photodetectors was compared with two different threshold levels. If the signal exceeded the upper threshold or fell to a level below the lower threshold then the presence of a speck was assumed, for example. The outputs from each of the threshold comparators were combined at OR gates in order to determine the presence of a single speck or, using a counter to decide if a strip of metal contained too many specks.
In all known sets of optical apparatus for scanning the surface of web material, even in those which use two different threshold levels in the analysing circuit, the resolution is limited by the physical size of the photodetectors. For an array of adjacent photodetectors, the optical resolution is equal to the distance between the centers of adjacent detectors. If the light from the detected area of the surface is made to diverge before it reaches the array of photodetectors, a greater resolution could in theory be obtained. However, accurate measurements of the optical transmission factor of a web of material can only be obtained using parallel, non-divergent beams of light, and this requires that the photodetectors are as close together as are the regions on the surface which they detect. The situation can be improved with the use of optical fibers or fiber bundles, which allow the photodetectors to be stationed remotely from the surface which they detect. Nevertheless, the resolution of the system is still limited by the physical separation of adjacent optical fibres.